1. Field of the Invention
The present invention relates to the manufacture of plastic bags or packages on a form, fill and seal (FFS) machine, particularly a vertical form, fill and seal (VFFS) machine, from a sheet of thermoplastic material, wherein each plastic bag or package includes a reclosable plastic zipper comprising a pair of mutually interlocking zipper profiles. Specifically, the present invention comprises a method for continuously and sequentially forming bags or packages having such zippers disposed in a direction parallel to that of the filling tube of the FFS machine, and, consequently, in the direction in which the thermoplastic sheet progresses on the FFS machines during the production of the bags or packages. More specifically, the method concerns the attachment of a reclosable plastic zipper to the inside of a loop in a tube formed from the sheet of thermoplastic material during the production and filling of the bags or packages on a form, fill and seal machine.
2. Description of the Prior Art
The present invention relates to improvements in the package-making art and may be practiced in the manufacture of thermoplastic bags and packages of the kind that may be used for various consumer products, but which are particularly useful for food products which must be kept in moisture- and air-tight packages, free from leakage until initially opened for access to the product contents, which packages are then reclosable by zipper means to protect any remainder of the product therein.
The indicated art is fairly well-developed, but nevertheless remains susceptible to improvement contributing to increased efficiency and cost-effectiveness.
One problem that hampers the production of packages from continuous zipper-equipped sheet material is the difficulty in forming a longitudinal loop in a tube made from a sheet of thermoplastic material on a form, fill and seal machine, and in attaching a continuous reclosable plastic zipper to the inside surface thereof during the production of plastic bags or packages, the loop having the reclosable plastic zipper ultimately becoming the mouth of the bag or package. The difficulty, exacerbated by the high speeds at which form, fill and seal machines are operated, lies in the creasing and tearing of the thermoplastic sheet material during the formation of the loop.
U.S. Pat. No. 5,046,300 discloses an attempt at a solution to this problem, wherein packaging film is formed into a tubular form about a forming tube. Subsequently, the tubular shaped packaging film is advanced along the length of the forming tube and over a product fill tube having a circumference less than that of the forming tube. The packaging film is deformed to conform to the circumference of the product fill tube. Excess packaging film, made available due to the difference between the circumferences of the forming tube and product fill tube, is formed into a loop. A reclosable profile element is guided into the loop and adhered to the inner surface of the loop. The deformation required to conform the packaging film to the circumference the product fill tube has proved to be difficult to achieve in a controlled fashion in practice, and has been the source of many operating problems caused by the creasing and tearing of the sheet at high machine speeds.
The present invention represents an advance over that disclosed in U.S. Pat. No. 5,046,300 in that the loop, in which the reclosable plastic zipper is ultimately to be disposed, is formed in the sheet of thermoplastic material before it is wrapped around the fill tube of a form, fill and seal machine.